Arrangement for the protection of actuating electromagnets operated by alternating current against overheating



Feb. 21, 1967 G. WIDL 3,305,808

ARRANGEMENT FOR THE PROTECTION OF ACTUATING ELECTROMAGNETS OPERATED BYALTERNATING CURRENT AGAINST OVERHEATING Filed May 18, 1965 2Sheets-Sheet 1 FIG! INVENTOR Garlarc! Feb. 21, 1967 ARRANGEMENT FOR THEPR OPERATED BY ALTERN G. WIDL OTECTION OF ACTUATING ELECTROMAGNETS ATINGCURRENT AGAINST OVERHEATING Filed May 18, 1965 2 Sheets-Sheet 2 INVENTORCV' bin my L/ United States Patent B 9 Claims. Cl. 335-243 The presentinvention relates to an arrangement for the protection of actuatingelectromagnets operated by alternating current against overheating.

Actuating electromagnets, or control magnets, as for instance used inhydraulic arrangements for controlling the position of a valve member orthe like include a yoke of cylindrical or cup-shaped configurationcarrying a mag net winding and a movable armature connected to themember to be moved and attracted into the winding to a greater or lesserextent during energizing of the latter to change thereby the position ofthe member which is to be controlled by the electromagnet.

Electromagnets which are operated by alternating current draw at thebeginning of their working stroke a greater current than at the end ofthe working stroke in which the air gap between armature and yoke isreduced or in which the armature may abut against the yoke, whereby theimpedance of the magnet winding of the electromagnet greatly increases.If during operation of such an actuating magnet, which for instance maybe used to control a hydraulic arrangement, the armature due to jammingof the member connected thereto cannot be moved, the magnet winding willdraw, when energized, constantly a great current which will lead to anoverheating of the magnet and possibly destruction of the winding.

It is one object of the present invention to overcome this disadvantagein the operation of actuating electromagnets which are operated byalternating current.

It is an additional object of the present invention to provide for anarrangement including an actuating electromagnet which is operated byalternating current and in which overheating of the electromagnet ispositively prevented.

It is an additional object of the present invention to provide for anarrangement of the aforementioned type which is constructed ofrelatively few and simple parts so that the arrangement can bemanufactured at reasonable cost and will stand up trouble-free underextended use.

With these objects in view, the arrangement according to the presentinvention for the protection of actuating electromagnets operated byalternating current against overheating, mainly comprises support meanshaving an abutment face, an electromagnet including a yoke normallysupported on the abutment face of the support means and being slidablyguided by the support means movable away from and towards the abutmentface, and a magnet winding carried by the yoke and adapted to beconnected to a source of alternating current for energizing theelectromagnet. The arrangement includes further an armature separated byan air gap from the yoke to be moved along a working stroke, when themagnet is energized by an alternating current, whereby the current drawnby the winding is greater at the beginning and smaller at the end of theworking stroke, connecting means engaging the armature and being adaptedto be connected to an element to be moved by the armature duringmovement of the latter along the working stroke, and spring meansoperatively connected to the yoke and arranged and constructed toyieldably press the yoke against the abutment face of the support meanswith a force which at any position of the magnet and the armaturerelative to each other is smaller than the magnetic force in the airgap, but greater than the force to be exerted by the magnet to moveunder normal operating conditions the armature through its workingstroke, whereby when after energizing of the winding the armature isprevented from moving along the working stroke due to jamming or thelike of the element connected thereto, the yoke and the winding carriedthereby may move against the force of the spring means relative to thearmature to a position in which the current drawn by the winding isreduced to prevent thereby overheating of the latter. The aforementionedspring means may be constructed so that the spring force changes in anon linear manner with the change of the position of the yoke and thespring means may for instance be in the form of one or a plurality ofconical coil springs. To assure that in the moment of energizing themagnet winding the full force developed by the magnet Will act on thearmature, it is preferred to provide some damping means to dampen themovement of the yoke or to add additional weight to the yoke to increasethe inertia thereof.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawings, inwhich:

FIG. 1 is a longitudinal cross section of one embodiment of thearrangement according to the present invention;

FIG. 2 is a partial cross sectional view of an arrangement similar tothat shown in FIG. 1 but showing a different spring;

FIG. 3 is a longitudinal cross section through another embodimentaccording to the present invention; and

FIG. 4 is a transverse cross section taken along the line IV-IV of FIG.3.

Referring now to the drawings, and more specifically to FIG. 1 of thesame, it will be seen that the arrangement for the protection of anactuating electromagnet operated by alternating current againstoverheating comprises support means, shown in FIG. 1 as a substantiallycylindrical hollow housing 10, having in the region of the bottom endthereof an annular abutment face 10. Arranged within the housing 10 isan electromagnet including a substantially cup-shaped yoke 11 having abottom wall 11 and at its end opposite the bottom wall an inwardlyextending rim portion 18 having an upper annular face extendingsubstantially normal to the axis of the yoke, and an annular magnetwinding located in the yoke between the bottom wall 11' and the rimportion 18. The winding 12 is adapted to be connected to a source ofalternating current in a manner not shown in the drawing. The yoke 11 isslidably guided in the housing 10 movab'ly. away from and toward theabutment face 10'. The connection of the magnet winding 12 to the sourceof alternating current has to be made in such a manner so as not toimpede any movement of the yoke 11 and the winding carried thereby inaxial direction. The arrangement includes further a T-shaped armature 16having a stem portion extending through the rim portion 18 partly in thecylindrical space defined by the winding 12 and having outside the yokea transverse portion 17 projecting to opposite sides of the stem portionspaced from the upper face of the rim portion 18 of the yoke 11.Connecting means in form of a rod 19 are slidably guided in a centralbore of the bottom wall 11' of the yoke and engage with the upper endthereof the bottom face of the stem portion of the armature 16. Thelower end of the rod 19 is adapted to be connected in any suitablemanner to an element, not shown in the drawing, to be moved duringenergizing of the electromagnet.

When the winding 12 is energized by alternating current, the armature 16will under normal operating conditions be moved along its working strokein which the stem portion of the armature moves further into the winding12 and in which the transverse portion 17 approaches the rim portion 18of the yoke, whereby the impedance of the winding 12 increases so thatthe current drawn by the electromagnet is considerably reduced at theend of the working stroke of the armature.

If, for some reason, for instance due to jamming of the elementconnected to the rod 19, the armature cannot be moved when the windingis energized, the winding would draw continuously relatively greatcurrent which would lead to an overheating and destruction of thewinding.

To prevent such an occurrence the arrangement includes further springmeans, shown in FIG. 1 as two coil springs 13 and 14, which abut withthe lower ends against the upper surface of the yoke 11 and with theirupper ends against a cover which is by means not shown in the drawingfixedly connected to the housing 10. The springs 13 and 14 aredimensioned so as to press the bottom wall 11' of the yoke against theabutment face 10 of the housing 18 with a force which at any position ofthe yoke and the armature relative to each other is smaller than themagnetizing force between yoke and armature, but greater than the forcewhich under normal operating conditions is necessary to move the rod 19.

Due to the provision of the springs 13 and 14, and due to the specificdimensioning of the springs, the yoke 11 will, when the winding 12 isenergized and when the armature 16 is prevented from moving, moveagainst the force of the springs 13 and 14 relative to the stationaryarmature 16 so that the gap between the portions 17 of the armature 16and the portion 18 of the yoke 11 is reduced and the impedance of thewinding 12 thereby considerably increased so that the current passingthrough the winding will be considerably reduced and overheating of thewinding 12 thereby positively prevented. In dimensioning the springs 13and 14 it has to be also taken into account that the magnetizing forceacting between armature 16 and yoke 11, which has to move in the eventthe armature 16 is kept stationary, the yoke against the force of thespring means, has to overcome not only the force of the springs 13 and14 but has to overcome also the inertia of the yoke 11 and the winding12 carried thereby.

Instead of substantially cylindrical coil springs 13 and 14, as shown inFIG. 1, it is also possible to use.substantially conical coil springsbetween the cover 15 and the yoke 11 and one such conical coil spring 14is shown in FIG. 2. It is to be understood that the embodiment onlypartially shown in FIG. 2 corresponds in all other aspects to theembodiment as shown in FIG. 1. A conical coil spring as shown in FIG. 2is advantageous since the relationship between deflection of such a coilspring and the force developed by the coil spring during increaseddeflection corresponds substantially to the increase of the magneticforce developed by the electromagnet during relative movement ofarmature and yoke relative to each other in a direction in which the airgap is reduced.

To assure thatin the moment of energizing the winding of theelectr-omagnet the full force developed by the magnet will act on thearmature and the rod 19 connected thereto, it is advantageous to delayany possible movement of the yoke by damping means or to increase theinertia of the yoke by adding additional weight thereto.

Such an arrangement is shown in FIGS. 3 and 4, and this arrangement mayinclude a closed substantially cylindrical housing 20 having a bottomwall 20' and a top wall 20" which is formed at the inner surface thereofwith a central projecting portion which may have the shape of aspherical segment as shown in FIG. 3. Arranged within the housing 20 isa substantially U-shaped yoke 11" having a bottom portion and at itsupper end a pair of inwardly extending portions. A magnet winding 12 iscarried by the yoke 11" between the aforementioned portionsthereof.Fastened to the bottom portion of the yoke 11 is a substantiallycylindrical piston 28 which is provided at its cylindrical surface witha sealing ring 30 which sealingly engages the inner cylindrical surfaceof the housing 28. A movable armature 16' having a central stern portionextending into the winding 12 and a transverse portion outside the yoke11" and projecting with opposite ends beyond the stem portion of thearmature spaced from the inwardly extending portions of the yoke.Connecting means in form of a rod 19 abut with an upper end thereofagainst the bottom surface of the stem portion of the armature 16' andthe rod 19 extends with clearance through a bore 32 formed through thebottom portion of the yoke 11" and through the piston 28 and with alower portion thereof through a bore 34 in the bottom wall 20' of thehousing. The space within the housing about the aforementioned elementslocated therein is filled with a damping fluid 26, for instance oil.Appropriate sealing means, not shown in the drawing, are provided in thebore 34 about the bottom portion of the rod 19 to prevent any oil whichcan pass through the clearance between rod 19 and the bore 32 fromflowing out of the housing 20.

The arrangement shown in FIGS. 3 and 4 includes also spring meansoperatively connected to the yoke 11 for resiliently holding the latter,respectively the piston 28 connected thereto against the bottom wall 20'of the housing. The spring means are shown in FIG. 3 as a leaf spring 22connected by means of a screw or the like to the spherical projection onthe top wall 20" of the housing so as to project with lateral portionsthereof freely from the connected portion. The opposite ends of thespring 22 are preferably slightly curved, as shown in FIG. 3. A pair ofrods 24 extend parallel to the axis of the armature 16 and the oppositeends of the rods 24 are respectively located in the curved ends of thespring 22 and in appropriate cavities at the upper end of the yoke 11"to transmit the spring force to the latter. The spring 22 is dimensionedso as to develop a force of the same magnitude as described inconnection with FIG. 1 and for the same purpose as set forth above.Obviously, instead of the leaf spring 22 shown in FIG. 3, a pair of coilsprings as shown in FIG. 1 or conical coil springs as shown in FIG. 2could also be used in the arrangement illustrated in FIGS. 3 and 4.

The arrangement illustrated in FIGS. 3 and 4 will operate insubstantially the same manner as described'in connection with FIG. 1. Indimensioning the spring 22 it has to be taken into account that when thearmature 16' is prevented from moving during energizing of the winding12', the magnetic force developed by the magnet has not only to overcomethe pressure of the spring 22 to move the yoke 11 to a position in whichthe inwardly extending portions of the yoke 11" will engage thetransverse portion of the armature 16', but that the magnetic force hasalso to overcome the resistance of the damping arrangement and theincreased inertia of the yoke 11" due to the weight of the piston 28attached thereto. During upward movement of the yoke 11", oil in thehousing 20 will move from the space above the piston 28 through theclearance between the rod 19 and the bore 32 into the graduallyenlarging space below the piston 28.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofoverheating protection for actuating electromagnets operated byalternating current differing from the types described above.

While the invention has been illustrated and described as embodied in anarrangement for the protection of an actuating electromagnet operated byalternating current against overheating, it is not intended to belimited to the details shown, since various modifications and structuralchanges may be made without departing in any way from the spirit of thepresent invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. Arrangement for the protection of actuating electromagnets operatedby alternating current against overheating comprising, in combination,support means having an abutment face; an electromagnet including a yokenormally supported on said abutment face of said support means and beingslidably guided by said support means movably away from and toward saidabutment face, and a magnet Winding carried by said yoke and adapted tobe connected to a source of alternating current for energizing saidelectromagnet; an armature separated by an air gap from said yoke to bemoved along a working stroke when said magnet is energized by analternating current, whereby the current drawn by said winding isgreater at the beginning and smaller at the end of said working stroke;connecting means engaging said armature and being adapted to beconnected to an element to be moved by said armature during movement ofthe latter along said working stroke; and spring means operativelyconnected to said yoke and arranged and constructed to yieldably presssaid yoke against said abutment face with a force which at any positionof said magnet and said armature relative to each other is smaller thanthe magnetic force in said air gap but greater than the force to beexerted by said magnet to move under normal operating conditions saidarmature through its working stroke, whereby when after energizing ofsaid winding said armature is prevented from moving along said workingstroke due to jamming or the like of the element connected thereto, saidyoke and said winding carried thereby may move against the force of saidspring means relative to said armature to a position in which thecurrent drawn by said winding is reduced to prevent thereby overheatingof the latter.

2. Arrangement for the protection of actuating electromagnets operatedby alternating current against overheating comprising, in combination,support means having an abutment face; an electromagnet including a yokenormally supported on said abutment face of said support means and beingslidably guided by said support means movably away from and toward saidabutment face, and a magnet winding carried by said yoke and adapted tobe connected to a source of alternating current for energizing saidelectromagnet; an armature separated by an air gap from said yoke to bemoved along a working stroke when said magnet is energized by analternating current, whereby the current drawn by said winding isgreater at the beginning and smaller at the end of said working stroke;connecting means engaging said armature and being adapted to beconnected to an element to be moved by said armature during movement ofthe latter along said working stroke; and conical coil spring meansoperatively connected to said yoke and arranged and constructed toyieldably press said yoke against said abutment face with a force whichat any position of said magnet and said armature relative to each otheris smaller than the magnetic force in said air gap but greater than theforce to be exerted by said magnet to move under normal operatingconditions said armature through its working stroke, whereby when afterenergizing of said winding said armature is prevented from moving alongsaid working stroke due to jamming or the like of the element connectedthereto, said yoke and said winding carried thereby may move against theforce of said spring means relative to said armature to a position inwhich the current drawn by said winding is reduced to prevent therebyoverheating of the latter.

3. Arrangement for the protection of actuating elec tromagnets operatedby alternating current against overheating comprising, in combination,support means having an abutment face; an electromagnet including a yokenormally supported on said abutment face of said support means and beingslidably guided by said support means movably away from and toward saidabutment face, and a magnet winding carried by said yoke andadapted tobe connected to a source of alternating current for energizing saidelectromagnet; an armature separated by an air gap from said yoke to bemoved along a working stroke when said magnet is energized by analternating current, whereby the current drawn by said winding isgreater at the beginning and smaller at the end of said working stroke;connecting means engaging said armature and being adapted to beconnected to an element to be moved by said armature during movement ofthe latter along said working stroke; spring means operatively connectedto said yoke and arranged and constructed to yieldably press said yokeagainst said abutment face with a force which at any position of saidmagnet and said armature relative to each other is smaller than themagnetic force in said air gap but greater than the force to be exertedby said magnet to move under normal operating conditions said armaturethrough its working stroke, whereby when after energizing of saidwinding said armature is prevented from moving along said Working strokedue to jamming or the like of the element connected thereto, said yokeand said winding carried thereby may move against the force of saidspring means relative to said armature to a position in which thecurrent drawn by said winding is reduced to prevent thereby overheatingof the latter; and damping means cooperating with said yoke for dampingthe movement thereof.

4. Arrangement for the protection of actuating electromagnets operatedby alternating current against overheating comprising, in combination,support means having an abutment face; an electromagnet including a yokenormally supported on said abutment face of said support means and beingslidably guided by said support means movably away from and toward saidabutment face, and a magnet winding carried by said yoke and adapted tobe connected to a source of alternating current for energizing saidelectromagnet; an armature separated by an air gap from said yoke to bemoved along a working stroke when said magnet is energized by analternating current, whereby the current drawn by said winding isgreater at the beginning and smaller at the end of said working stroke;connecting means engaging said armature and being adapted to beconnected to an element to be rnoved by said armature during movement ofthe latter along said working stroke; spring means operatively connectedto said yoke and arranged and constructed to yieldably press said yokeagainst said abutment face with a force which at any position of saidmagnet and said armature relative to each other.is smaller than themagnetic force in said air gap but greater than the force to be exertedby said magnet to move under normal operating conditions said armaturethrough its working stroke, whereby when after energizing of saidwinding said armature is prevented from moving along said Working strokedue to jamming or the like of the element connected thereto, said yokeand said winding carried thereby may move against the force of saidspring means relative to said armature to a position in which thecurrent drawn by said winding is reduced to prevent thereby overheatingof the latter; and weight means fixed to said yoke for increasing theinertia thereof.

5. Arrangement for the protection of actuating electromagnets operatedby alternating current against overheating comprising, in combination,hollow housing means having in the region of one end thereof an inwardlyextending shoulder forming an abutment face and a cover at the other endthereof; an electromagnet including a substantially cup-shaped yokehaving a bottom wall normally supported on said abutment face and at anend thereof opposite said bottom wall an inwardly extending rim portionhaving an outer annular face, and a magnet winding located in said yokebetween said bottom wall and said rim portion thereof and adapted to beconnected to a source of alternating current, said yoke being slidablyguided in said housing movably away from and toward said abutment face;a T-shaped armature separated by an air gap from said yoke and having astem portion extending through said rim portion of said yoke into saidwinding and having outside said yoke a transverse portion projecting toopposite sides of said steam portion spaced from said outer annular faceof said rim portion, said armature moving, when said electromagnet isenergized, along a working stroke in which the transverse portionthereof moves toward said annular face whereby the current drawn by saidwinding is reduced as said transverse portion approaches said annularface; connecting means engaging said stem portion of said armature andbeing adapted to be connected to an element to be moved by said armatureduring movement of the latter along said working stroke; and springmeans abutting with opposite ends thereof against said cover of saidhousing means and said outer annular face of said rim portion of saidyoke, said spring means being dimensioned so as to press said yokeagainst said abutment face with a force which at any position of saidmagnet and said armature relative to each other is smaller than themagnetic force in said air gap but greater than the force to be exertedby said magnet to move under normal operating conditions said armaturethrough its working stroke, whereby when after energizing of saidwinding said armature is prevented from moving along said working strokedue to jamming or the like of the element connected thereto, said yokeand said winding carried thereby may move against the force of saidspring means relative to said armature to a position in which thecurrent drawn by said winding is reduced to prevent thereby overheatingof the latter.

6. Arrangement for the protection of actuating electromagnets operatedby alternating current against overheating comprising, in combination,hollow housing means having in the region of one end thereof an inwardlyextending shoulder forming an abutment face and a cover at the other endthereof; an electromagnet including a substantially cup-shaped yokehaving a bottom Wall formed with a central bore therethrough and beingnormally supported on said abutment face and at an end thereof oppositesaid bottom wall an inwardly extending rim portion having an outerannular face, and a magnet winding located in said yoke between saidbottom wall and said rim portion thereof and adapted to be connected toa source of alternating current, said yoke being slid- .ably guided insaid housing movably away from and toward said abutment face; a T-shapedarmature separated by an air .gap from said yoke and having a sternportion extending through said rim portion of said yoke into saidwinding and having outside said yoke a transverse portion projecting toopposite sides of said stem portion spaced from said outer annular face'of said rim portion, said armature moving, when said electromagnet isenergized, along a working stroke in which the transverse portionthereof moves toward Said annular face whereby the current drawn by saidwinding is reduced as said transverse portion approaches said annularface; connecting means including a rod slidably guided in said centralbore of said bottom wall of said yoke and engaging with one end thereofthe end of said stem portion of said armature facing said bottom walland being adapted to be connected at the other end thereof to an elementto be moved by said armature during movement of the latter along saidworking stroke; and spring means abutting with opposite ends thereofagainst said cover of said housing means and said outer annular face ofsaid rim portion of said yoke, said spring means being dimensioned so asto press said yoke against said abutment face with a force which at anyposition of said magnet and said armature relative to each other issmaller than the magnetic force in said air gap but greater than theforce to be exerted by said magnet to move under normal operatingconditions said armature through its working stroke, whereby when afterenergizing of said winding said armature is prevented from moving alongsaid working stroke due to jamming or the like of the element connectedthereto, said yoke and said winding carried thereby may move against theforce of said spring means relative to said armature to a position inwhich the current drawn by said winding is reduced to prevent therebyoverheating of the latter.

7. Arrangement for the protection of actuating elee tromagnets operatedby alternating current against overheating comprising, in combination, aclosed housing having a cylindrical wall, a top wall and a bottom wallformed with a central bore therethrough; an electromagnet including ayoke having a transverse bottom portion facing said bottom wall and apair of inwardly extending portions at one end thereof opposite saidtransverse bottom portion, and a magnet winding carried by said yokebetween said transverse bottom portion and said inwardly extendingportions thereof; a piston fixed to said transverse bottom portion ofsaid yoke and carrying a sealing ring slidably engaging said cylindricalwall of said housing, said piston and said transverse portion of saidyoke being formed with a central bore therethrough aligned with saidbore through said bottom wall of said housing, said piston having abottom face normally abutting against said bottom Wall of said housingand said yoke being slidably guided by said piston in said housingmovably away from and toward said bottom wall of the latter; a T-shapedarmature separated by an air gap from said yoke and having a sternportion extending into said winding and outside said yoke a transverseportion projecting to opposite sides of said stem portion spaced fromsaid inwardly extending portions of said yoke, said armature moving,when said electromagnet is energized, along a working stroke in whichsaid transverse portion thereof moves toward said inwardly extendingportions of said yoke, whereby the current drawn by said winding isreduced as said transverse portion approaches said inwardly extendingportions; connecting means including a rod extending with clearancethrough said bore of said transverse portion of said yoke and saidpiston and in a sealed manner through said central bore in said bottomwall of said housing, said rod engaging with one end thereof saidstem-shaped portion of said armature and being adapted to be connectedat the other end thereof to an element moved by said armature duringmovement of the latter along said working stroke; a damping fluidfilling the space in said housing about said magnet and said armature;and spring means operatively connected to said yoke and arranged andconstructed to resiliently press said piston carried by said yokeagainst said bottom wall of said housing with a force which at anyposition of said magnet and said armature relative to each other issmaller than the magnetic force in said air gap, but greater than theforce to be exerted by said magnet to move under normal operatingconditions said armature through its working stroke, whereby when afterenergizing of said winding said armature is prevented from moving alongsaid working stroke due to jamming or the like of the element connectedthereto, said yoke, said winding and piston carried thereby may moveagainst the force of said spring means relative to said armature to aposition in which the current drawn by said winding is reduced toprevent thereby overheating of the latter, during movement of said yokeand said piston carried thereby said damping fluid may pass through saidclearance between said rod and the inner surface of said bore throughsaid piston from one to the other side of the latter.

8, An arrangement as set forth in claim 7, wherein said spring means isa leaf spring fixed at a substantial-1y central portion thereof to saidtop wall and having a pair of 10 housing, and including a pair of rodsrespectively arranged spaced from opposite ends of said transverseportion of said T-shaped armature and engaging with opposite endsthereof said free ends of said leaf spring and said one end of saidyoke.

9. An arrangement as set forth in claim 7, wherein said damping fluid isoil.

References Cited by the Examiner UNITED STATES PATENTS 1,194,132 3/1916Brown 335-240 2,692,354 10/1954 Fisher 335-240 BERNARD A. GILHEANY,Primary Examiner.

opposite free ends spaced from said top wall of said 15 G. HARRIS,Assistant Examiner.

1. ARRANGEMENT FOR THE PROTECTION OF ACTUATING ELECTROMAGNETS OPERATEDBY ALTERNATING CURRENT AGAINST OVERHEATING COMPRISING, IN COMBINATION,SUPPORT MEANS HAVING AN ABUTMENT FACE; AN ELECTROMAGNET INCLUDING A YOKENORMALLY SUPPORTED ON SAID ABUTMENT FACE OF SAID SUPPORT MEANS AND BEINGSLIDABLY GUIDED BY SAID SUPPORT MEANS MOVABLY AWAY FROM AND TOWARD SAIDABUTMENT FACE, AND A MAGNET WINDING CARRIED BY SAID YOKE AND ADAPTED TOBE CONNECTED TO A SOURCE OF ALTERNATING CURRENT FOR ENERGIZING SAIDELECTROMAGNET; AN ARMATURE SEPARATED BY AN AIR GAP FROM SAID YOKE TO BEMOVED ALONG A WORKING STROKE WHEN SAID MAGNET IS ENERGIZED BY ANALTERNATING CURRENT, WHEREBY THE CURRENT DRAWN BY SAID WINDING ISGREATER AT THE BEGINNING AND SMALLER AT THE END OF SAID WORKING STROKE;CONNECTING MEANS ENGAGING SAID ARMATURE AND BEING ADAPTED TO BECONNECTED TO AN ELEMENT TO BE MOVED BY SAID ARMATURE DURING MOVEMENT OFTHE LATTER ALONG SAID WORKING STROKE; AND SPRING MEANS OPERATIVELYCONNECTED TO SAID YOKE AND ARRANGED AND CONSTRUCTED TO YIELDABLY PRESSSAID YOKE AGAINST SAID ABUTMENT FACE WITH A FORCE WHICH AT ANY POSITIONOF SAID MAGNET AND SAID ARMATURE RELATIVE TO EACH OTHER IS SMALLER